Hydrocracking is a process which has achieved widespread use in petroleum refining for converting various petroleum fractions to lighter and more valuable products, especially distillates such as jet fuels, diesel oils and heating oils. Hydrocracking is generally carried out in conjunction with an initial hydrotreating step in which the heteroatom-containing impurities in the feed are hydrogenated without a significant degree of bulk conversion. During this initial step, the heteroatoms, principally nitrogen and sulfur, are converted to inorganic form (ammonia, hydrogen-sulfide) and these gases may be removed prior to the subsequent hydrocracking step although the two stages may be combined in cascade without interstage separation as, for example, in the Unicracking-JHC process and in the moderate pressure hydrocracking process described in U.S. Pat. No. 4,435,275.
In the second stage of the operation, the hydrotreated feedstock is contacted with a bifunctional catalyst which possesses both acidic and hydrogenation/dehydrogenation functionality. In this step, the characteristic hydrocracking reactions occur in the presence of the catalyst. Polycyclic aromatics in the feed are hydrogenated, and ring opening of aromatic and naphthenic rings takes place together with dealkylation. Further hydrogenation may take place upon opening of the aromatic rings. Depending upon the severity of the reaction conditions, the polycyclic aromatics in the feed will be hydrocracked to paraffinic materials or, under less severe conditions, to monocylic aromatics as well as paraffins. Naphthenic and aromatic rings may be present in the product, for example, as substituted naphthenes and substituted polycyclic aromatics in the higher boiling products, depending upon the degree of operational severity.
The bifunctional catalyst used in the hydrocracking process typically comprises a metal component which provides the hydrogenation/dehydrogenation functionality and a porous, inorganic oxide support provides the acidic function. The metal component typically comprises a combination of metals from Groups IVA, VIA and VIIIA of the Periodic Table (IUPAC Table) although single metals may also be encountered. Combinations of metals from Groups VIA and VIIIA are especially preferred, such as nickel-molybdenum, cobalt-molybdenum, nickel-tungsten, cobalt-nickel- molybdenum and nickel-tungsten-titanium. Noble metals of Group VIIIA especially platinum or palladium may be encountered but are not typically used for treating high boiling feeds which tend to contain significant quantities of heteroatoms which function as poisons for these metals.
The porous support which provides the acidic functionality in the catalyst may comprise either an amorphous or a crystalline material or both. Amorphous materials have significant advantages for processing very high boiling feeds which contain significant quantities of bulky polycyclic materials (aromatics as well as polynapthenes) since the amorphous materials usually possesses pores extending over a wide range of sizes and the larger pores, frequently in the size range of 100 to 400 Angstroms (.ANG.) are large enough to provide entry of the bulky components of the feed into the interior structure of the material where the acid-catalyzed reactions may take place. Typical amorphous materials of this kind include alumina and silica-alumina and mixtures of the two, possibly modified with other inorganic oxides such as silica, magnesia or titania.
Zeolitic crystalline materials, especially the large pore size zeolites such as zeolites X and Y, have been found to be useful for a number of hydrocracking applications since they have the advantage, as compared to the non-zeolitic materials, of possessing a greater degree of activity, which enables the hydrocracking to be carried out at lower temperatures at which the accompanying hydrogenation reactions are thermodynamically favored. In addition, the zeolitic crystalline catalysts tend to be more stable in operation than the non-zeolitic materials such as alumina. The zeolitic crystalline materials may, however, not be suitable for all applications since even the largest pore sizes in these materials, typically about 7.4 .ANG. in the X and Y zeolites, are too small to permit access by various bulky species in the feed. For this reason, hydrocracking of residuals fractions and high boiling feeds has generally required a non-zeolitic catalyst of rather lower activity. Although it would be desirable, if possible, to integrate the advantages of the non-zeolitic and the zeolitic crystalline material in hydrocracking catalysts and although the possibility of using active supports for zeolitic crystalline materials has been proposed, the difference in activity and selectivity between the non-zeolitic and zeolitic crystalline materials has not favored the utilization of such catalysts.
The crystalline hydrocracking catalysts based on zeolites such as zeolites X and Y generally tend to produce significant quantities of gasoline boiling range materials (approximately 330.degree. F.-, 165.degree. C.-) materials as product. Since hydrocracked gasolines tend to be of relatively low octane and require further treatment as by reforming before the product can be blended into the refinery gasoline pool, hydrocracking is usually not an attractive route for the production of gasoline. On the other hand, it is favorable to the production of distillate fractions, especially jet fuels, heating oils and diesel fuels since the hydrocracking process reduces the heteroatom impurities characteristically present in these fractions to the low level desirable for these products. The selectivity of crystalline aluminosilicate catalysts for distillate production may be improved by the use of highly siliceous zeolites, for example, the zeolites possessing a silica: alumina ratio of 50:1 or more, as described in U.S. Pat. No. 4,820,402 (Partridge et al), but even with this advance in the technology, it would still be desirable to integrate the characteristics of the amorphous materials with their large pore sizes capable of accommodating the bulky components of typical hydrocracking feeds, with the activity of the zeolite catalysts.
While the considerations set out above apply mostly to fuels hydrocracking processes, they will also be relevant in greater or lesser measure to lube hydrocracking. In the lube hydrocracking process, which is well established in the petroleum refining industry, an initial hydrocracking step is carried out under high pressure in the presence of a bifunctional catalyst which effects partial saturation and ring opening of the aromatic components which are present in the feed. The hydrocracked product is then subjected to dewaxing in order to reach the target pour point since the products from the initial hydrocracking step which are paraffinic in character include components with a relatively high pour point which need to be removed in the dewaxing step.
In theory, as well as in practice, lubricants should be highly paraffinic in nature since paraffins possess the desirable combination of low viscosity and high viscosity index. Normal paraffins and slightly branched paraffins e.g. n-methyl paraffins, are waxy materials which confer an unacceptably high pour point on the lube stock and are therefore removed during the dewaxing operations in the conventional refining process described above. It is, however, possible to process waxy feeds in order to retain many of the benefits of their paraffinic character while overcoming the undesirable pour point characteristic. A severe hydrotreating process for manufacturing lube oils of high viscosity index is disclosed in Developments in Lubrication PD 19(2), 221-228, S. Bull et al, and in this process, waxy feeds such as waxy distillates, deasphalted oils and slack waxes are subjected to a two-stage hydroprocessing operation in which an initial hydrotreating unit processes the feeds in blocked operation with the first stage operating under higher temperature conditions to effect selective removal of the undesirable aromatic compounds by hydrocracking and hydrogenation. The second stage operates under relatively milder conditions of reduced temperature at which hydrogenation predominates, to adjust the total aromatic content and influence the distribution of aromatic types in the final product. The viscosity and flash point of the base oil are then controlled by topping in a subsequent redistillation step after which the pour point of the final base oil is controlled by dewaxing in a solvent dewaxing (MEK-toluene) unit. The slack waxes removed from the dewaxer may be reprocessed to produce a base oil of high viscosity index. Processes of this type, employing a waxy feed which is subjected to hydrocracking over an amorphous bifunctional catalyst such as nickel-tungsten on alumina or silica-alumina are disclosed, for example, in British Patents Nos. 1,429,494, 1,429,291 and 1,493,620 and U.S. Pat. Nos. 3,830,273, 3,776,839, 3,794,580, and 3,682,813.
In lube processes of this kind, the catalyst is, like the fuels hydrocracking catalyst, typically a bifunctional catalyst containing a metal hydrogenation component on an amorphous acidic support. The metal component is usually a combination of base metals, with one metal selected from the iron group (Group VIIIA) and one metal from Group VIA of the Periodic Table, for example, nickel in combination with molybdenum or tungsten. The activity of the catalyst may be increased by the use of fluorine, either by incorporation into the catalyst during its preparation in the form of a suitable fluorine compound or by in situ fluoriding during the operation of the process, as disclosed in GB 1,390,359.
Although the lube hydrocracking processes using an amorphous catalyst for the treatment of the waxy feeds has shown itself to be capable of producing high V.I. lubricants, it is not without its limitations. The major process objective in lube hydrocracking (LHDC) is to saturate the aromatic components in the feed to produce saturated cyclic compounds (naphthenes) or, by ring opening of the naphthenes, paraffinic materials of improved lubricating properties. This requires the hydrogenation activity of the catalyst to be high. There is no corresponding requirement for a high level of cracking activity since no major change in boiling range is required or even desirable: the amount of material in the lube boiling range, typically 650.degree. F.+, should be maintained at the maximum level consistent with the degree of ring opening required to furnish a lube product of the desired quality. This combination of requirements has typically led to the use of LHDC catalysts with high metals loadings, particularly for base metal combinations with Group VIA metals such as tungsten: commercial LHDC catalysts currently available have typical nickel loadings of about 5 percent but the tungsten loading may be in the range of 10 to 25 percent.
We have now found that a solid acid catalyst comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal may be used as the basis for hydrocracking catalysts. It is an object of the present invention to provide a hydrocracking process using a high activity catalyst. It is a further object of the present invention to provide a hydrocracking process using such a catalyst with products having high isoparaffin/normal paraffin ratios and a low C.sub.1 -C.sub.2 yield.